1. Field of the Invention
The present invention generally relates to a developing apparatus for use in electrophotography, and more particularly, to a developing sleeve rotary type magnetic brush developing apparatus which includes a developing sleeve having an internal magnet member with magnetic poles.
2. Description of Related Art
Conventionally, in the electrophotographic developing method, there have been proposed many processes, among which an important method most widely applied to practical applications is a two or dual-component developing method. In this dual-component developing method, the magnetic brush process which transports toner mixed in a magnetic carrier by a magnetic field for developing has many superior points such as high image quality, low cost and stability in characteristics, and wide range of selection of materials, etc., and is employed as a standard practice for visualizing electrostatic latent images.
For the magnetic carrier, magnetic particles or powder having particle diameters slightly larger than toner such as iron particles, triiron tetroxide (magnetite), ferrite and the like are employed. Particles of ferrite, magnetite, etc. are made into powder of the required particle size by grinding after kneading and solidification or by sintering after granulation with a water base binder. Most of the ordinary carrier is in the form of spherical shape about 30 to 100 .mu.m or indefinite particle shape.
When mixed into carrier, toner is electrically charged through contact with carrier face, and forms a uniform mixture by being attracted to carrier, and is restricted by the magnetic field of the magnet member of the developing roll so as to be transported to an electrostatic latent image holding member such as a photoreceptor or the like. The developing is generally effected at the portion where the magnetic field of the developing roll is perpendicular to the surface of the electrostatic latent image holding member. The magnetic carrier particles are formed in a line contacting each other along the direction of the magnetic field, and forms brush bristles (magnetic brush) at the magnetic field vertical portion on the internal magnetic poles of the magnet member in the developing roll. At such brush bristle portion, gaps are formed in a direction perpendicular to the face, and through the gaps, toner are moved by the actin of the electric field to effect the developing.
Although the dual-component developing method described above has various advantages referred to earlier as compared with a mono-component developing method and the like, there is a problem in that it is low in the developing efficiency, and difficult to maintain the toner concentration in the carrier to be constant and uniform.
When the toner concentration in the developing material is lowered, the toner charge amount is increased, with reduction of the image density. Conversely, upon increase of the toner concentration in the developing material, the toner charge amount is reduced, and fogging or soiling tends to appear, with an increase of toner scattering. Moreover, since toner is not naturally dispersed as in a liquid, non-uniformity in density is apt to appear, with a consequent deterioration in the image quality. Accordingly, constant and uniform control of the toner concentration is very important.
Generally, since the optimum amount of addition of toner to the carrier is less than 10%, the toner concentration fluctuates even by a slight mount of toner consumption in the developing, and the image characteristic tends to vary to a large extent. Furthermore, for the supply of toner, mixing can not be achieved merely by addition, but a sufficient stirring is required. If the stirring is not enough, toner aggregation remains without being crushed, resulting in fogging, soiling or the like which may give rise to image noises.
The reason why the two-component developing apparatus tends to be heavy in weight and large in size, with a high cost, is considered to be attributable to the fact that a large space is required for the toner replenishment and mixing and the toner concentration is difficult to be controlled.
In order to solve the problems as described above, there has conventionally been proposed, for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 59-111664 or Tokkaisho No. 61-80280, a toner automatic replenishing system in which magnetic carrier is caused to be attracted onto a developing sleeve including a magnet member therein so as to adjust the developing material by supplying charged toner to said carrier from a toner supply roll. By the above known replenishing system, although a stirring section may be dispensed with for compact size of the developing apparatus, it is still difficult to completely solve the problem for stability in the charging and supply of the developing material, and the problem related to the image quality and fogging, etc.
One of the reasons for such disadvantages is due to the fact that the toner concentration automatic control effect has largely depended not on a perfect electrostatic process, but, on a toner exchange balance process between the developing material supply roll and the developing roll. Therefore, a sufficient automatic concentration adjusting effect can not be achieved unless the toner exchange amount is considerably increased. However, if the above exchange amount is large, the toner with a small charge amount on the supply roll is to be supplied and collected by a large amount at all times, thus generating fogging. Moreover, owing to the necessity to control the charged toner layer on the supply roll to constant thickness by a restriction blade, thickened toner layer is scraped off, and by the erasing function with electrical discharging which takes place this time, the toner particles are subjected to large non-uniform charge, thereby to form the aggregate, which also results in the image deterioration. Additionally, there are also such problems as fusion of toner onto the supply roll and variation of image characteristics, etc. due to particle diameter selecting effect.